1. Technical Field
The present invention refers to a microelectromechanical structure comprising distinct parts mechanically connected through a translation-to-rotation motion converting assembly.
2. Description of the Related Art
As is known, optical devices formed by microelectromechanical structures (MEMs) are currently studied for guiding laser light beams. These optical devices in general comprise switches that have the function of deflecting the laser light beams and are controlled by electronic circuitry, preferably integrated circuits, associated to the devices.
FIG. 1 is a schematic representation of an optical device 1 of the indicated type, which comprises a first optical transmission element 2, a second optical transmission element 3, and a third optical transmission element 4. The optical transmission elements may be of any type, for example optical fibers, waveguides, etc. The second optical transmission element 3 is arranged at 90xc2x0 with respect to the first optical transmission element 2, whereas the third optical transmission element 4 is arranged at preset angle, different from 90xc2x0, with respect to the first optical transmission element 2.
An optical switch 7 is arranged between the optical transmission elements 2-4 to direct an incident light ray, which traverses the first optical transmission element 2, selectively towards the second optical transmission element 3 or the third optical transmission element 4. The optical switch 7 comprises a mirror element 8 and a control structure (not shown) which rotates the mirror element 8 between a first position (indicated by the solid line) and a second position (indicated by the dashed line). In the first position, the mirror element 8 is arranged at 45xc2x0 with respect to the first optical transmission element 2 and the second optical transmission element 3, so that an incident ray 9, supplied by the first optical transmission element 2, is reflected towards the second optical transmission element 3 (reflected ray 10 represented by a solid line), whilst in the second position, the mirror element 8 is arranged at an angle different from 45xc2x0 with respect to the first optical transmission element 2 and the second optical transmission element 3, and the incident ray 9 is reflected towards the third optical transmission element 4 (reflected ray 11 represented by a dashed-and-dotted line).
The third optical transmission element 4 may not be present. In this case, the optical switch 7 operates as an on/off switch, which enables or disables transmission of the light ray 9.
Rotation of the mirror element 8 is obtained by applying a twisting moment lying in the plane of the mirror element 8, which is suspended from a bearing structure through spring elements (two or four, according to the number of desired freedom degrees). At present, the twisting moment necessary for rotating the mirror element 8 is generated in two ways: via electrostatic forces acting directly on the mirror element 8, or via a mechanical conversion assembly which converts a translation of a linear actuator into a rotation.
FIG. 2 is a schematic representation of an electrostatic actuation system. The mirror element 8 is formed by a platform 15 of semiconductor material suspended from a frame 18 through two spring elements 17a extending in the X direction starting from two opposite sides of the platform 15. The frame 18 is in turn supported by a first wafer 16 of semiconductor material through two spring elements 17b extending in the Y direction starting from two opposite sides of the platform 15. The spring elements 17a, 17b of each pair are aligned to one another and are sized in order to be substantially rigid to tension/compression and to be compliant to torsion, so as to form pairs of axes of rotation of the platform 15. Specifically, the spring elements 17a define an axis of rotation parallel to the X axis, and the spring elements 17b define an axis of rotation parallel to the Y axis. In the vicinity of its four comers, the platform 15 has, on the underside, electrodes 20 facing corresponding counterelectrodes 21 arranged on a second wafer 22, arranged underneath. When appropriate differences of potential are applied between one pair of electrodes 20 and the respective counterelectrodes 21, one side of the platform 15 is subjected to an attractive force (arrows F in FIG. 2), which generates a twisting moment M about two opposed spring elements (in this case the spring elements 17a), so causing rotation of the platform 15 in the desired direction and with the desired angle.
FIG. 3 is a schematic representation of a mechanical actuation system. Also in this case, the mirror element 8 is formed by a platform 15 made of semiconductor material supported by the first wafer 16 through spring elements 17a, 17b and through the frame 18.
On the underside of the platform 15 is arranged an element having the shape of a frustum of a pyramid integral with the platform 15 and defining a lever 25. The lever 25 is engaged by four projecting elements, in this case four walls 26 extending vertically upwards starting from a plate 27 and each arranged perpendicular to the adjacent walls 26. The plate 27 (illustrated in greater detail in FIG. 4) is suspended from a frame 30 through two spring elements 28 extending in the X direction starting from two opposite sides of the plate 27. The frame 30 is in turn supported by the second wafer 22 through two spring elements 31 extending in the Y direction starting from two opposite sides of the frame 30. The spring elements 28, 31 are sized in such a way as to be compliant, respectively, in the Y direction and in the X direction, and to be more rigid to rotation.
According to what is illustrated in FIG. 5, the plate 27 is suspended above a cavity 34 present in one protection layer 36 (for instance, a layer of silicon dioxide) which overlies a substrate 35 belonging to the second wafer 22 and in which there are formed integrated components belonging to the control circuitry. The plate 27 is conveniently made in a third wafer 37 bonded between the first wafer 16 and the second wafer 22.
The plate 27 may translate as a result of the electrostatic attraction between actuating electrodes 38, 39. For this purpose, on the underside of the plate 27 there are present mobile electrodes 38 facing fixed electrodes 39 formed on the bottom of the cavity 34. In use, the mobile electrodes 38 and the fixed electrodes 39 are biased in such a way as to generate a translation of the plate 27 in the X direction or in the Y direction or in a vector combination of the two directions, exploiting the elastic compliance of the spring elements 28 and 31 in both directions.
The walls 26-lever 25 ensemble form a conversion assembly 40 that converts the translation of the plate 27 into a rotation of the platform 15, as illustrated in FIG. 5, which illustrates the effect of a displacement in the X direction of the plate 27. This displacement determines, in fact, a corresponding displacement of the walls 26, in particular, of the wall 26 on the left in FIG. 5; this wall 26, by engaging the lever 25, draws it towards the right, thus determining a rotation of the platform 15 by an angle xcex8 about the spring elements 17b (one of which may be seen in FIG. 3), which are represented by the axis 17 in FIG. 5.
The linear actuation of the plate 27 thus enables rotation of the platform 15 about the axes defined by the spring elements 17a or 17b or both, so enabling the platform 15 to assume a plurality of angular positions that may be controlled through the actuation electrodes 38, 39.
The described conversion assembly 40 is affected by hysteresis, which limits the precision in the control of the platform 15 and causes part of the movement of the plate 27 to be ineffective. In fact, to ensure the engagement of the lever 25 with the walls 26 also in presence of misalignments between the first wafer 16 and the third wafer 37 and to take into account the fabrication tolerances as regards the height of the walls 26, as well as the shape of the latter and of the lever 25, the pairs of facing walls 26 are arranged at a greater distance than necessary for engaging the lever 25, as indicated by the solid line and, in an exaggerated way, in detail in FIG. 6. As a result, in the first part of the movement of the plate 27, it may happen that the wall 26, which should interact with the lever 25, fails to engage the lever 25 immediately and does not cause rotation of the platform 15 at once. For example, in FIG. 6, for a displacement of the plate 27 in the direction of the arrows, the rotation of the platform 15 starts only when the wall 26 on the left arrives in contact with the lever 25 and the plate 27 has displaced by the amount xcex94X. The same applies, in the illustrated example, for a displacement of the plate 27 in the direction opposite to that of the arrow, even though in general the amount of displacement in one direction or the other is different and not correlated.
The same problem of hysteresis described above afflicts in general all the microstructures formed by a translating part and a rotating part connected by an assembly for converting the translation into a rotation, the assembly having a play or hysteresis as a result of the tolerances and fabrication imprecisions.
An embodiment of the invention provides a microelectromechanical structure having an motion converting assembly that is free from the problem referred to above.
The microelectromechanical structure is usable in an optical switch for directing a light beam towards one of two light guide elements. The structure includes: a rotatably movable mirror element; an actuator that is movable with translatory motion; and a motion conversion assembly arranged between the mirror element and the actuator. The motion conversion assembly includes a projection integral with the mirror element and elastic engagement elements integral with the actuator and elastically loaded towards the projection. The elastic engagement elements are formed by metal plates fixed to the actuator at one of their ends and engaging the projection with an abutting edge countershaped with respect to the projection.
A process for manufacturing a microelectromechanical structure is further provided, including the steps of forming a first part which is rotatably movable, the first part including a projection, forming a second part that is movable with translatory motion, the second part including elastic engagement elements, and assembling the first and second parts, in that, during the assembling step, the elastic engagement elements automatically and elastically engage the projection.